Currently commercial secondary batteries include a nickel cadmium battery, a nickel hydrogen battery, a nickel zinc battery, a lithium secondary battery, etc. Among them, lithium secondary batteries are attaining much attention because they are freely charged or discharged due to rare occurrence of a memory effect and have a very low self-discharge rate and a high energy density, compared with nickel-based secondary batteries.
Such lithium secondary batteries generally use a lithium-based oxide and a carbon material as a positive electrode active material and a negative electrode active material, respectively. Lithium secondary batteries include an electrode assembly, and an outer casing, namely, a battery case, that seals and accommodates the electrode assembly together with an electrolyte. The electrode assembly includes a positive electrode plate and a negative electrode plate respectively coated with such a positive electrode active material and such a negative electrode active material, and a separator interposed between the positive and negative electrode plates.
In general, lithium secondary batteries may be classified into can-type secondary batteries including an electrode assembly built in a metal can, and pouch-type secondary batteries having an electrode assembly built in a pouch of an aluminum laminate sheet.
Recently, secondary batteries are being widely used in not only small devices, such as portable electronic devices, but also medium and large sized devices, such as cars or power storage devices. In such battery packs, a large number of secondary batteries may be electrically connected to each other in order to increase the capacity and the output. Pouch-type secondary batteries are being more widely used due to advantages, such as easy stacking and a light weight.
Pouch-type secondary batteries may be generally manufactured by injecting an electrolyte into a pouch casing having an electrode assembly accommodated therein and sealing the pouch casing.
FIG. 1 is an exploded perspective view of a pouch-type secondary battery according to the related art, and FIG. 2 is an assembled perspective view of the pouch-type secondary battery of FIG. 1.
As shown in FIGS. 1 and 2, the pouch-type secondary battery may include an electrode assembly 20 and a pouch casing 30 that accommodates the electrode assembly 20.
The electrode assembly 20 basically includes a positive electrode plate, a negative electrode plate, and a separator between the positive and negative electrode plates, and may be accommodated in an inner space I formed in the pouch casing 30. The pouch casing 30 may include an upper pouch 31 and a lower pouch 32. Sealing parts S may be provided on respective outer circumferential surfaces of the upper pouch 31 and the lower pouch 32, respectively, and may be attached to each other to seal the inner space I in which the electrode assembly 20 is accommodated.
At least one positive electrode tab 21 and at least one negative electrode tab 22 may extend from the positive electrode plate and the negative electrode plate, respectively. The positive electrode tab 21 and the negative electrode tab 22 may be coupled with a plate-shaped electrode lead 10. In other words, the positive electrode tab 21 and the negative electrode tab 22 may be coupled with a plate-shaped positive electrode lead 11 and a plate-shaped negative electrode lead 12, respectively. The positive electrode lead 11 and the negative electrode lead 12 are partially exposed to the outside of the pouch casing 30, thereby providing an electrode terminal that can be electrically connected to an external structure of the secondary battery, for example, another secondary battery or an external device.
One of the most important issues for such a secondary battery is ensuring safety. If safety of a secondary battery is not properly ensured, this may lead to not only damage of the secondary battery but also accidents, such as an electric shock, a fire, and an explosion, thereby causing life damage and property damage.
In particular, secondary batteries may generate gas therein for various reasons, such as overcharge, overdischarge, and a short circuit. For example, in the case of lithium secondary batteries, an electrolyte may be dissolved during overcharge, and a large amount of gas may be generated. As such, when gas is generated in a secondary battery, an inner pressure of the secondary battery increases, and thus the secondary battery may swell. The swelling may not only cause damage of the secondary battery but also lead to explosion or fire-catching of the secondary battery. In particular, when a secondary battery explodes or catches fire, a device including the secondary battery may be damaged, and accordingly a user using the device may be injured. Moreover, when an electrolyte inside the secondary battery leaks due to damage of the outer casing of the secondary battery, a problem, such as a short circuit or an electric shock, may occur. In addition, hybrid cars, electric cars, and power storage devices, which are driven by batteries, have greatly large outputs and capacities, and thus damage thereof due to such internal generation of gas may become serious.